Table of contents

The second-order correlation function for light scattered at resonance from a two-level atom is calculated. The measurement of intensity correlations employing either homodyne or heterodyne techniques provides a novel approach for the extraction of linewidth and splitting frequencies.

X-ray spectra, in which radiative electron capture, both to the outer (n>or=2) shells and to the K shell of the projectile, is observed, have been collected for 30-65 MeV ¹⁶O ions incident upon various gas targets. The measured capture ratio is in reasonable agreement with theoretical estimates based on a 1/n³ capture cross section dependence.

Experimental differential cross sections for excitation of the 2³S and the 2³P states in He by electron collision are reported in the region of collision energies from 50 to 500 eV and from 50 to 200 eV respectively, in a relatively wide range of scattering angles. The angular distribution of scattered electrons for the 2³S excitation shows a complicated variation as a function of the collision energy, especially for energies lower than 100 eV. The differential cross section (DCS) shows a remarkable increase at the smallest scattering angles in addition to a broad maximum which appears at a few tens of degrees, even for energies larger than 100 eV. In contrast, the DCS for the 2³P excitation shows a simple trend of variation. Each curve has a distinct maximum, which moves to smaller angles as the collision energy increases.

It is shown that the experimentally obtained energy dependence of the total ionization cross section of Ar by He(2¹S) can be explained in terms of a saturation of the autoionization width Gamma (R) in the small R region. The associative ionization cross section is calculated using the previously determined molecular ion potential curve. The agreement with experiment is satisfactory.

Positions and relative intensities of Stark components of lines are altered in very dense plasmas because free electrons partially screen not only the Coulomb interaction between the radiating ion and perturbing ions, but also the Coulomb interaction within the radiating ion itself. This latter plasma-screening perturbation and the perturbation caused by the ion-field gradient (quadrupole interaction) are comparable. The first-order corrected Stark patterns for Lyman- alpha lines of several ions are presented in order to demonstrate the necessity of considering both of these perturbations and the fine-structure interaction simultaneously in the calculation of line profiles for high-density plasmas.

Transitions 2s²p^t-2s2p^t+1-2p^t+2 in iron spectra Fe XX, Fe XXI and Fe XXII have been identified using a laser plasma source. Wavelengths and relative intensities have been measured. The laboratory data obtained are used for some revision of published solar line identifications.

Results of an extended study of the rule of constancy of Delta nu /R for constant Z for spin doublets of X-ray non-diagram (satellite) lines are reported for the first time. The rule is shown to be valid for a number of L and M spin doublets of X-ray non-diagram lines. Behaviour of the spin doublets of X-ray diagram and non-diagram lines seems to be similar in this regard.

The resonance functions observed in sensitized fluorescence are derived for the magnetic resonance of an oriented or aligned atomic system. The functions are seen to be modified forms of those observed in direct fluorescence, but with an apparent anisotropy in the relaxation processes, even in an isotropic environment.

The theory of two-photon ionization including spin-orbit coupling effects is discussed. General expressions are given for total generalized cross sections, angular distributions and photoelectron spin-polarization. The effect of light polarization is taken into account. The calculation of atomic matrix elements is based on quantum-defect theory. Numerical results for selected cases (some of experimental interest) of two-photon ionization of alkali atoms are also presented.

The 1s to 2s and 1s to 2p total and differential cross sections for electron impact excitation of atomic hydrogen are calculated from 13 eV to 300 eV. The 1s-2s-2p close-coupling approximation is used for the low partial waves and the unitarized Born approximation for the high partial waves. At the highest energy up to 500 partial waves are included. The calculated differential cross sections are in quite good agreement with the measurements of Williams and Willis (1975).

A systematic investigation of the ejected-electron spectra of Ar is reported for incident electron energies 25-300 eV. The positions of several singly- and doubly-excited autoionizing states in the energy range 25-54 eV have been measured and compared, where possible, with the results of other workers. Using a Rydberg-Ritz analysis, many higher unobserved members of the ns ¹S and ³S, np ¹P and ³P and nd ¹D series have been calculated. Evidence demonstrating the existence in certain instances of a strong post-collision interaction between the scattered and ejected electrons is presented and discussed.

Electron impact differential and integrated cross sections for the excitation of the autoionizing level in zinc (3d to 4p) and cadmium (4d to 5p) are calculated using the first Born approximation (FBA) and the modified first Born approximation (MFBA). The orbital approximation has been employed. The ground-state orbital (nd, n=3 for Zn and n=4 for Cd) is represented by analytical SCF orbitals as obtained by Clementi (1967). The excited orbitals (4p and 5p) have been approximated by hydrogenic orbitals with appropriate effective Z.

An exact parabolic model for perturbed symmetric resonance is developed and solved. To supplement the Zwaan-Stueckelberg theory (1932) a comparison-equation method is adopted. It reduces the problem to parabolic cylinder function factors for which the corres ponding diabatic crossing is classically inaccessible. Appeal to the author's previous strong-coupling expansions for parabolic cylinder functions of large order (1972) and argument is made to reinforce the view that the adiabatic treatment gives the more precise picture. Both semiclassical and impact-parameter derivations are given. Previous formulae as reported by the author are proven (1971).

Second-order induction energies arising in the interaction between two hydrogen atoms have been calculated both for two ground-state atoms and for a ground and an excited (2s) state atom. The method employed is a rapidly convergent variational procedure with explicitly two-centre basis functions. The induction energies are very small at large internuclear separations, but they are not completely negligible in the binding regions.

The linewidth of carbonyl fluoride has been measured at room temperature (300K) using a double-modulation microwave spectrograph. Sixteen transitions have been studied for self-broadening cases. Six of these transitions have also been studied for the foreign-gas broadening in N₂, BrCN, OCS, CH₃CN and CHCl₃ linewidths have also been calculated using Anderson-Tsao-Curnutte (1962) and Murphy-Boggs (1967) theories and compared with experimental results. Variation of the linewidth with the quantum number J has also been studied both theoretically and experimentally.

Gryzinski's impulse model is applied to the dissociative collision of an atom with a diatomic molecule. The twofold differential cross section d² sigma /d cos theta d Delta E, where theta is the scattering angle of the atom and Delta E the energy transfer to the molecule, is given by a simple integral over binary encounter cross sections for scattering of the projectile atom by only one of the target atoms. Complete contour maps i.e. lines of constant d² sigma /d cos theta d Delta E values in Delta E( theta ) polar diagrams, are calculated for the systems D₂+O⁺ and HD+O⁺ at collision energies of 25 eV and 39.3 eV, respectively (in the centre-of-mass system). Comparison is made with corresponding experimental results.

Dipole oscillator strengths for absorption and for formation of parent and fragment ions of N₂ and CO in the energy range of 10-60 eV have been obtained, using techniques of energy-loss electron spectroscopy and electron-ion coincidence. A quantitative collection has been made of the fragment ions regardless of their initial kinetic energy. An analysis of the data, leads to partial oscillator strengths for a variety of dissociative processes involving excitations of two-electron states and deep valence-shell states. In addition, the branching of some of these dissociative states of CO⁺ over the various fragmentation channels is reported. Evidence is presented for the validity of the 'shake-up' model which has been used in calculating relative intensities of the C² Sigma ⁺ states with respect to those of the B² Sigma ⁺ states of the two molecules.

A simple, approximate method for calculating low-energy electron-molecule scattering cross sections using only the results of a basis set diagonalization of the molecular Hamiltonian is presented. The method is based on the approximate conservation of orbital angular momentum in collisions between slow electrons and molecules lacking a permanent dipole moment (low l spoiling). Results are presented for e^--H₂ and e^--N₂ in the static-exchange approximation.